1. Field of the Invention
The invention relates to a method and device for reducing a magnetic unidirectional flux component in the core of a three-phase transformer.
2. Description of the Related Art
Electrical transformers, such as those used in energy distribution networks, can be subject to the unwanted injection of a direct current into a primary winding or secondary winding. The injection of a direct current of this kind, hereinafter also referred to as the DC component can, for example, originate from electronic structural components such as are used nowadays to control electrical drives or even for power-factor compensation. Another cause could be “geomagnetically induced currents” (GIC).
In the core of the transformer, a DC component results in a flux component that superposes the alternating flux. This results in an asymmetrical control of the magnetic material in the core and is associated with a series of drawbacks. Even a direct current of a few amperes can cause local heating in the transformer, which can impair the lifetime of the winding insulation. A further unwanted effect is increased noise emission during operation of the transformer because modern transformer cores have very high magnetic conductivity and, hence, even small currents are sufficient to cause the transformer core to reach saturation in a half-cycle of the alternating current.
Various apparatuses that work actively and passively to reduce the operating noise of a transformer are known. According to WO 2012/041368 A1, an electric voltage is induced into the compensation winding and employed for the compensation of the disruptive magnetic unidirectional flux component. Here, an electronic switching unit generates a compensation current, where the switching-on of the switching unit follows a prespecified switching strategy, for example, via phase-angle control. Here, a thyristor switch is connected in series with a current-limiting reactor to introduce the compensation current into the compensation winding.
The above-described measures reduce the thermal loading of the winding of the transformer and also reduce losses and noises. The device for reducing a magnetic unidirectional flux component can be implemented with relatively simple means using discrete and/or programmable modules. No energy store, such as a battery or a capacitor, is required for the generation of the compensation current. Instead, the energy for the generation of the compensation current is taken directly from the compensation winding. Due to its simplicity, the circuit arrangement is very reliable and is well suited for the low-maintenance long-term operation of a transformer in an energy distribution network. The field of operation includes both transformers in the low- or medium-voltage range and very powerful transformers (power transformers, HVDC (high-voltage DC transmission) transformers). Neither the size nor safety-relevant apparatuses or other design criteria of the transformer are influenced unfavorably by the use of the system.
To enable the device for reducing a magnetic unidirectional flux component to be produced with inexpensive standard components and to keep this device within the officially defined voltage limits, such as with the voltage limits defined in the Low Voltage Directive (Directive 2006/95/EC), the permitted voltage induced in the compensation winding is limited in practice to a specific value, according to the Low Voltage Directive to 690 V.
Therefore, since the turn-to-turn voltage of an existing compensation winding increases with the nominal power of the three-phase transformer, the number of turns of the compensation winding can be kept very low. This can mean that with this low number of turns it is only possible to achieve a relatively weak magnetic field, which is not sufficient for any significant reduction of the magnetic unidirectional flux component. If the number of turns of the compensation winding were increased, in many applications, the prespecified maximum voltage would be exceeded. In this case, the device for reducing a magnetic unidirectional flux component could not be used.